The present invention relates to a mass spectrometer and, more particularly, to an improvement in those mass spectrometers which have functions of electron impact ionization (EI) and atmospheric pressure ionization (API).
Recently, a remarkable progress has been made in trace-impurities analyses of gases, which consists of efficiently ionizing a sample at an atmospheric pressure and introducing the resulting ions into a mass spectrometer through a fine aperture.
By way of reference material, analyzing methods of the above mentioned kind have been described in articles entitled "New Picogram Detection System Based on a Mass Spectrometer with an External Ionization Sources at Atmospheric Pressure" by E. C. Horning et al. (Analytical Chemistry, Vol. 45, No. 6, May 1973), and in "Subpicogram Detection System for Gas Phase Analysis Based upon Atmospheric Pressure Ionization (API) Mass Spectrometry" by E. C. Horning et al. (Analytical Chemistry, Vol. 46, No. 6, May 1974), as well as in the specification of Japanese Patent Application No. 78293/1974 entitled "Atmospheric Ionization Mass Spectrometer", filed by the present inventors. In these methods, primary ions are generated by a radioisotope or by an electric discharge and then a sample is ionized by ion molecule reactions. Thus, these methods are superior to other conventional methods employing electron impact ionization in that they can afford an ionization efficiency higher by about one figure than those provided by the other methods thereby to ensure higher sensitivity and provide a simpler spectrum which is easy to read, due to less decomposition of the sample.
Referring to FIG. 1 which shows a conventional atmospheric pressure ionization mass spectrometer having a radioisotope as means for effecting the ionization, a sample is efficiently ionized in an ionization region 1, by ion molecule reactions with a radioisotope 5 or with reactant ions generated by the radioisotope 5.
The ion molecule reactions are considered to take place in the following process. In general, the sample reaches the ionization region while it being suspended or contained by a carrier gas such as N.sub.2, O.sub.2, Ar or air. In the ionization region, the carrier gas is ionized by the radioisotope and the resultant ions produce an ion molecule reaction with water remaining in the carrier gas to produce reactant ions such as H.sub.2 O.sup.+ or (H.sub.2 O).sub..eta. H.sup.+. Then, the reactant ions perform an ion molecule reaction with the trace amounts of molecules to be detected to ionize the latter. The generated ions are introduced through a fine aperture 4 into a mass spectrometric analysis region 3 in which a high vacuum is established. The analysis is carried out in this region by means of a quadrupole mass spectrometer or a spectrometer employing a sector magnetic field and is finally detected by a detector 6. An EI ion source for mass calibration 7 which is an electron impacting source in the illustrated arrangement is disposed at the upstream side of the mass spectrometric analysis region 3.
In this known arrangement, an normal electron impact ionization spectrum is not obtained because the sample-carrying gas necessarily passes through the ionization region 1 where the radioisotope 5 is located. In addition, since the differential pumping is performed through only one fine aperture 4, it is necessary to make the diameter of the aperture small for example, 20.mu.m, so that the clogging of the aperture or other troubles are likely to occur.
In the atmospheric ionization, only molecules having a low ionization potential and molecules which tend to be ionized by the addition of H.sup.+ are selectively ionized with high efficiency through the ion molecule reaction, while substances having a high ionization potential and substances which are less likely to be ionized by the H.sup.+ addition are not kept ionized even when the carrier gas is rich in such substances. Therefore, it is not possible to make the best use of the high sensitivity of API if only atmospheric pressure ionization is used solely. A combination of the atmospheric pressure ionization with the electron impact ionization is preferred.
FIG. 2 shows an arrangement which is also a conventional one but improved to overcome the above described problems. The ionization by radioisotope is replaced by an ion source using a corona discharge, while two-stage differential pumping is adopted to allow use of a larger diameter fine aperture.
Referring to FIG. 2, an electrode 8 for the corona discharge is effective to cause an ionization and anion molecule reaction in the ionization region 1. The ions of the molecules to be measured pass through the aperture 4 to a mediate pressure region 2 which is evacuated to about 1 Torr by means of evacuation means (now shown). Since the ion molecule reaction takes place also in this mediate pressure region, this region 2 may be referred to as an ion molecule reaction region. The ions are then introduced into the mass spectral region 3, through a second fine aperture 9.
The diameters of the first and the second fine apertures 4 and 9 are, for example, 100.mu.m and 200.mu.m, respectively. The mediate pressure region 2 is provided with ion focusing auxiliary electrodes 10 for producing a electric field effective to converge and focus the ions on the second aperture 9. In this arrangement, it is possible to alternately conduct an analysis by atmospheric pressure ionization and an analysis by ionization by another means, such as electron impact ionization at the upstream side of the mass spectral region. This means that all kinds of gases may be conveniently ionized to be analyzed, without deteriorating the sensitivity in analyzing molecules having a low ionization potential and molecules which are more likely to be ionized by addition of H.sup.+ (proton transfer). The adoption of two-stage differential pumping allows the use of a larger fine aperture, which conveniently ensures that the problem of clogging of the aperture will be avoided.
However, this improved arrangement disadvantageously requires provision of a plurality of ion focusing auxiliary electrodes in the mediate pressure region 2. In addition, this arrangement is not effective as a separator, when used in the electron impact ionizing mode, since no substantial concentration of heavy molecule is expected as the molecules travel from the sample gas inlet port to the ion source located at the upstream side of the mass spectral region. To explain this in more detail, when the arrangement of FIG. 2 utlizes electron impact ionization, the density of the molecules to be measured at the ionization region is as low as that at the gas inlet port, so that the analyzing sensitivity is insufficiently low, although the arrangement is apparently designed to perform the electron impact ionizing function.
In order to overcome this problem, it has been proposed to adopt a jet separator which has been commonly used in combining a gas chromatograph with a mass spectrometer (GC-MS). The jet separator is usually capable of injecting a sample from a gas chromatograph working almost at atmospheric pressure to a mass spectrometer working at a vacuum of about 10.sup.-5 Torr. In addition, the jet separator is capable of selectively injecting a heavier sample carried by a lighter carrier such as He, owing to the radial diffusion velocity differential of the jetted molecules, which differential is attributable to the difference in weights of the molecules, thus effecting the "concentration" of the sample.
This solution in which the jet separator is found, however, is also inconvenient in that ionized molecules are hardly injected into the mass spectrometer when the apparatus is used in the atmospheric pressure ionization mode.